This invention relates to a microelectronic package comprising an integrated circuit component mounted on a substrate by solder bump interconnections. More particularly, this invention relates to such microelectronic package wherein the solder bump interconnections are formed of a tin-base, lead-free solder alloy that includes a controlled copper addition.
In the manufacture of microelectronic packages, it is known to attach an integrated circuit component to a substrate, such as a printed circuit board or the like, by a plurality of solder bump interconnections. Each solder bump interconnection joins a first bond pad on the substrate to a second bond pad on the integrated circuit component. The integrated circuit component may be an integrated circuit die affixed to a ceramic or polymeric carrier that is in turn attached to the board. Alternately, the component may be an integrated circuit die that is inverted and attached directly to the board. In either event, the component comprises a surface facing the substrate whereon the second bond pads are disposed in a pattern superposable onto the first bond pads.
To form the interconnections, a solder bump is deposited onto each bond pad of the component. A common method comprises placing a preformed microsphere of solder onto each pad and heating to reflow the solder to form the bump. Alternately, solder may be deposited by electroplating or other suitable metal deposition process. The component with the solder bumps is assembled with the substrate so that each bump rests against a corresponding bond pad on the board. The assembly is heated to melt and reflow the solder. During reflow, the component is supported by the molten solder. This is accompanied by collapse of the component toward the board until the weight of the component is balanced by the surface tension of the molten solder. Concurrently, the molten solder wets the bond pads, which wetting is essential to forming a strong solder bond. Upon cooling, the solder resolidifies to complete the interconnections. The solder bump interconnections not only physically attach the component to the board, but also electrical connect the pads on the component to the pads on the substrate for conducting electrical signals to or from the component for processing.
Common solder is formed of tin-lead alloy. During reflow, copper from the bond pad tends to dissolve into the molten solder. Copper dissolution is limited by the solubility of copper at the reflow temperature and is typically less than 0.5 percent. Upon cooling, the dissolved copper forms intermetallic precipitates that tend to reduce the strength of the interconnections. For this reason, it has been desired to minimize copper content in tin-lead solder alloy.
This invention contemplates a microelectronic package comprising an integrated circuit component mounted on a printed circuit board or the like by a plurality of solder bump interconnections formed from a lead-free, tin-base solder alloy that contains a significant copper addition. In general, the solder alloy in accordance with this invention is composed between about 2 and 8 weight percent copper, preferably between about 3 and 5 weight percent. Optionally, the alloy may contain silver or other metallic additive to enhance mechanical properties. Lead is present only as impurity, preferably less than 0.1 weight percent.
In one aspect of this invention, a method is provided that includes initially forming solder bumps having the desired copper addition onto the bond pads of the integrated circuit component. The component is assembled with the substrate, and the assembly is heated and cooled to reflow the solder alloy and form the desired solder bump interconnections. It is found that the tin-copper solder alloy exhibits a high surface tension effective to reduce collapse of the component toward the substrate during reflow. It is also found that, in the absence of lead, precipitation of copper intermetallics is inhibited, so that the resulting solder bump interconnections exhibit mechanical strength comparable to prior tin-lead interconnections despite the significant copper addition.